A social network is a social structure made up of individuals (referred to as “nodes”) who are connected to one another via one or more relationships, such as friendship, kinship, common interest, and so forth. A social networking service is an online service, platform, or site that enables users to participate in a social network and build connections with other users.
In some social networking services connections are directional; for example, a first user may be a “follower” of a second user, while the second user may or may not be a follower of the first user. In some other social networking services, connections are always bi-directional; for example, if a first user is a “friend” of a second user, then the second user is also a friend of the first user (i.e., both the first user and second user are “friends”). Users of a bi-directionally-connected social networking service can be thought of as being directionally connected; for example, if Alice and Bob are friends in a bi-directionally-connected social networking service, one could also say that Alice is a follower of Bob and Bob is a follower of Alice.
Social networking services are playing an increasing role in the distribution of content (e.g., web pages, pictures, video clips, etc.) among users. In particular, a user may share a particular content item with some or all of his connections, each of whom may in turn share the content item with some or all of their connections, and so forth. When a shared content item propagates rapidly and quickly reaches a large number of users, the content item may be said to have great “virality,” or in popular usage, “to have gone viral.”